1. Field of the Invention
The present invention relates to a hearing aid that improves clarity by minimizing the sense that sounds instantly become louder, eliminating the metallic ring to sounds, and so forth.
2. Description of the Related Art
The process by which sound waves are recognized by our auditory system is generally considered to be extremely complex, but to summarize this process, sound waves travel through a conducting system consisting of the external ear canal, the eardrum, the auditory ossicle, the cochlea, hair cells, nerves, and brain cells, where the sound waves are recognized. Within this conducting system, the external ear canal and eardrum are called the outer ear, the eardrum and auditory ossicle are called the middle ear, and the cochlea and hair cells are called the inner ear.
A hearing impairment therefore occurs when any of the functions is diminished in this conducting system, and the symptoms will vary, as will the method of dealing with them, depending on which function is diminished and to what extent.
The typical form of senile deafness is an overall decrease in function, including brain function, making it difficult to hear weak sounds.
FIG. 7 is a graph of equisignal curves of the loudness of sound in humans with normal hearing. The horizontal axis is the frequency (Hz), and the vertical axis is the sound pressure level (dB). Sound pressure level will hereinafter be abbreviated as SPL.
The curves in the graph are known as Fletcher-Manson curves, and the hatched area in the figure indicates the distribution of acoustic energy in a typical conversation. The dashed line labeled xe2x80x9cminimum audible levelxe2x80x9d is a curve corresponding to a human with normal hearing, but in the elderly this is higher on the graph, as with the curve indicated by the dashed line labeled xe2x80x9csenile deafness minimum audible level.xe2x80x9d This senile deafness minimum audible level varies from person to person, so the curve in the graph should be viewed as just an example.
As can be seen from the acoustic energy distribution in a typical conversation, a person with senile deafness is only able to hear about half of the sounds in the voice spectrum which a person with normal hearing is able to hear, so even though the sounds may be perceptible, the hearer cannot make out the words.
With the example shown in the graph, if the acoustic level is raised about 50 dB by a hearing aid, the voice spectrum of conversation will be more or less reach the audible level, allowing the wearer to understand the words, but sounds of, say, 80 dB, which are encountered on an everyday basis, become 130 dB, which is so loud as to be uncomfortable.
The highest level that a person with normal hearing is able to stand is about 130 dB, and is said to be between 120 and 130 dB for a person who is hard of hearing, which would seem to be about the same, but in fact the level is often much lower.
FIG. 8 is a graph of the formants of Japanese vowels. The horizontal axis is the first formant (kHz), and the vertical axis is the second formant (kHz) (see Rika Nenpyo, p. 491, published by Maruzen, Nov. 30, 1985).
What FIG. 8 tells us is that for the Japanese vowels xe2x80x9cAxe2x80x9d, xe2x80x9cIxe2x80x9d, xe2x80x9cUxe2x80x9d, xe2x80x9cExe2x80x9d, and xe2x80x9cOxe2x80x9d to be clearly distinguished, for example, the second formant must be reliably transmitted with respect to the first formant.
FIG. 9 is a table of typical values for various sounds and their corresponding formant frequencies. According to this table, the second formant frequency varies between 1.5 and 7.7 times with respect to the first formant frequency, but if it is not reliably transmitted, the hearer cannot distinguish between A, I, U, E, and O.
In general, the level of the second formant is about 20 to 40 dB lower than the level of the first formant, so even if the first formant can be heard, it is difficult to hear the second formant, and to make matters worse, there is usually a dramatic drop in the perception of high frequencies with a person with senile deafness, as indicated by the dashed line in FIG. 7, and this makes it even more difficult to hear the second formant, in which case even though the person may be able to hear the first formant, he does not understand what is being said.
Because of the above situation, one thing conventional hearing aids had in common was that they raised the level of the second formant high enough to be audible, but while employing this means does indeed work fairly well with mild deafness, with more severe deafness the level of the first formant often exceeds 100 dB, which sounds loud to the wearer.
Raising the degree of amplification of high frequencies has been accomplished by using a tone control circuit, and while this is effective with persons of mild deafness, with a more severe case of deafness, if the frequency of the first formant is high, the first formant level can rise over 100 dB and become painful, and as a result the wearer hears a so-called ringing noise.
Automatic volume adjusting circuits are frequently used to keep the volume below 100 dB by immediately lowering the gain if a loud sound over 100 dB should come in. Various methods have been developed for shielding the wearer from fluctuations in sound level by optimizing the attack time and release time, but if someone should suddenly shout during a conversation, the level is lowered to the point that it sounds as if the sound source is far away, and this is particularly undesirable when listening to sounds through a stereo audio device because the sensation of a fixed position is lost and the location of the sound source seems to float around.
It is an object of the present invention to provide a hearing aid which amplifies voices so that they can be clearly understood but do not sound overly loud.
The hearing aid of the present invention is designed so that the gain of the second formant is raised without raising the gain of the first formant, which keeps the clarity of voices high without their sounding too loud. A state in which even the first formant cannot be heard is not under discussion here, in which case it is necessary to perform overall amplification so that the first formant can be heard, and raise the gain of the second formant.
The level of the first formant in conversation is usually about 50 to 60 dB, which is high, and even people with mild to moderate deafness can still hear adequately, but because the level of the second formant is about 20 to 40 dB lower than that of the first formant, voices will not seem too loud even if the second formant is boosted to about this same level.
Therefore, not raising the gain of the first formant and raising the gain of the second formant makes voices become clear, and since the gain of the first formant does not change, the voices do not sound loud.
FIG. 1 consists of graphs of the operating condition settings of the hearing aid pertaining to the present invention. The horizontal axis is frequency, and the vertical axis is the SPL. FIG. 1A shows the frequency spectrum related to the vowel xe2x80x9cIxe2x80x9d seen in FIG. 8, and FIG. 1B shows the frequency spectrum related to the vowel xe2x80x9cAxe2x80x9d seen in FIG. 8.
For example, if a person cannot hear sounds below an SPL of 50 dB, then, as is obvious from FIG. 1A, that person can only hear the first formant with the vowel xe2x80x9cIxe2x80x9d and cannot, tell which sound it is, further since he can faintly hear the second formant with the vowel xe2x80x9cAxe2x80x9d as shown in FIG. 1B, he can tell that the sound is xe2x80x9cAxe2x80x9d, although he will be uncertain if the voice is a little softer.
With the hearing aid pertaining to the present invention, as shown by the broken line in FIG. 1A and 1B, the first formant is not amplified, and just the second formant is amplified enough to reach the required level, thus bringing both the first formant and second formant within the audible range.
With the xe2x80x9cIxe2x80x9d sound in FIG. 1A, frequencies of the 350 Hz frequency of the first formant and higher are corrected by 6 dB/oct up to a maximum of 20 dB.
This correction strengthens the second formant (2.7 kHz, SPL of 42 dB) by 18 dB, bringing it up to SPL of 60 dB, so a person who cannot hear below an SPL of 50 dB can adequately catch the first and second formants and is able to tell that the sound is xe2x80x9cI.xe2x80x9d The corrected frequency spectrum is indicated by a one-dot chain line in FIG. 1A.
With the xe2x80x9cAxe2x80x9d sound in FIG. 1B, frequencies of the 1 kHz frequency of the first formant and higher are corrected by 6 dB/oct up to a maximum of 20 dB.
With the sound xe2x80x9cA,xe2x80x9d even without correction, a person who cannot hear below an SPL of 50 dB can tell that the sound is xe2x80x9cAxe2x80x9d if he pays close attention, since the second formant is 53 dB, but the level rises to SPL 57 dB with correction, which allows the sound to be heard more clearly. Again in FIG. 1B, the corrected frequency spectrum is indicated by a one-dot chain line.
A feature of the correction characteristics in the hearing aid of the present invention is that they change in relation to the change in the first formant frequency. In the past, when frequency characteristics were corrected by tone control or the like, the correction characteristics themselves did not change when the first formant changed.
For instance, when a conventional tone control is used to set the correction characteristics to match the frequency spectrum of the sound xe2x80x9cIxe2x80x9d seen in FIG. 1A (that is, the correction characteristics indicated by the broken line of FIG. 1A), and the wearer hears the sound xe2x80x9cAxe2x80x9d in this state, 1 kHz, which is the first formant of the sound xe2x80x9cAxe2x80x9d as shown in FIG. 1B, is strengthened by 10 dB, bringing the SPL of first formant up to 80 dB and making the sound xe2x80x9cAxe2x80x9d 10 dB louder than the sound xe2x80x9cI.xe2x80x9d This results in a so-called ringing noise because the degree of amplification for first formant rises along with the frequency of the first formant rises as the sound xe2x80x9cAxe2x80x9d.
Because the extent of hearing impairment can vary widely, correction of a hearing aid must be matched to the extent of impairment of the user, and therefore the amount of correction must be matched to the user, and cannot be fixed.
When correction is thus tailored to the extent of impairment of the user, if the user cannot hear even the first formant, then first of all amplification must be performed for all frequencies up to the level where the first formant can be heard, and then the corrective amplification for the second formant pertaining to the present invention must be performed.
The first and second formants described above are the minimum elements required to understand language, and useful information is also contained in the third, fourth, and subsequent formants, so reproducing these is also important, and since these are contained in substantially higher frequencies than the first formant, the correction pertaining to the present invention is effective with them as well.
The above description is focused primarily on language, but being able to hear frequencies over the first formant is effective for musical notes and all information obtained from sound waves and required in our daily lives, and makes it possible to obtain more information.
Because of the above, first aspect of the present invention is a hearing aid for amplifying an acoustic signals:
(1) comprising:
a controller for determining in real time a frequency band at the highest level of the acoustic signals through frequency analysis of the acoustic signals that vary over time, and for generating a control signal to raise a gain for signals of a higher frequency range than the frequency band at the highest level (such as an amplifier Q3, or a band-pass filter group 2 and a diode matrix 3 and a comparator 4, or a digital signal processor 13, or the like); and
a first amplifier, in which the control signal from said controller is inputted so that the frequency characteristics are varied, for amplifying the acoustic signals by increasing the gain for signals of the higher frequency range than the frequency band at the highest level (such as an amplifier system consisting of amplifiers Q1 and Q2, or a parametric equalizer 5, or a digital signal processor 13, or the like), or
(2) in (1) above, the controller comprising a second amplifier whose gain is a function of the frequency (such as the amplifier Q3), or
(3) in (1) above, the first amplifier, comprising an amplification apparatus (such as an amplification apparatus including amplifiers Q1 and Q2) in which a plurality of sub-amplifiers with different frequency characteristics, each capable of gain control, are connected in parallel, and the outputs of the plurality of sub-amplifiers are added together, or
(4) in (1) above, the controller comprising a band-pass filter group (such as the band-pass filter group 2), a diode matrix (such as the diode matrix 3), and a comparator group (such as the comparator group 4), or
(5) in (1) above, the first amplifier, comprising a parametric equalizer, or
(6) comprising:
an A/D converter provided on the side where the acoustic signals are inputted, for converting analog signals of the acoustic signals into digital signals (such as an A/D converter 12);
a digital signal processor for determining in real time a frequency band at the highest level of the digital signals through frequency analysis of the digital signals that are outputted from the A/D converter and vary over time, and then for generating a control signal for raising a gain for signals of a higher frequency range than the signal of the frequency band at the highest level, and then for amplifying the digital signals by increasing the gain for signals of the higher frequency range than the frequency band at the highest level, according to the control signal; and
a D/A converter for converting the digital signals outputted from the digital signal processor into analog signals (such as a D/A converter 14).
The adoption of the above structure results in a hearing aid which amplifies an input acoustic signals so that all sounds can be clearly understood but do not sound overly loud.
The second aspect of the present invention is a hearing aid for amplifying an input acoustic signals that vary over time comprising:
a control circuit for generating a control signal according to a first frequency band at the highest level of the input acoustic signals; and
an amplifier for amplifying the input acoustic signals so as to generate an output acoustic signals, wherein the amplifier has a frequency characteristic including a first gain region which has a constant gain for frequencies equal to or lower than the first frequency band, and a second gain region whose gain increases higher than the first gain region, according to frequency, for frequencies higher than the first frequency band; and in response to the control signal, an increase point between the first and second gain regions changes according to the first frequency band.
The frequency characteristic for the gain is dynamically controlled depending on the first frequency band at the highest level of the input acoustic signals so that the increase point between the flat gain region and the increasing gain region changes dynamically.